Perfectly Ripe Fruits | How Do They Do It?
There’s nothing like biting into perfectly ripe fruits, like a peach or a juicy apple. But how do these fruits make it all the way from where they’re grown, to us – and arrive at just the right time?
The science behind fruit ripening
Ripe fruits are softer, sweeter, and more brightly coloured.
Their colour change happens as chlorophyll (the pigment that makes plants green) breaks down, while other colourful pigments like anthocyanin and carotenoids accumulate. The fruits’ sweetness comes from starches inside the fruit breaking down into simpler sugars like glucose and fructose. And their softness comes from cell walls breaking down.1
Fruits fall into two categories in terms of ripening: either they will keep ripening by themselves once they are picked, or they won’t.
Those that continue ripening are known as climacteric and include apples, bananas, pears, apricots, peaches, blueberries and plums. Those that stop ripening once they’re picked are called non-climacteric, and include citrus fruits like lemons and limes, raspberries, strawberries, grapes, watermelon, and pineapple.2
Arriving perfectly ripe: It’s all about ethylene
Climacteric fruits produce a burst of ethylene gas, which acts as a plant hormone, when they start ripening. Non-climacteric fruits also produce ethylene, but they don’t release a spike of it like climacteric fruits do. They can, however, ripen further before they’re picked, if they are around ethylene produced by other fruits or an external source.3
Since an ethylene spike signals the start of ripening for climacteric fruits, dampening down the levels of ethylene around them can slow down the ripening process – allowing producers to store climacteric fruits (like apples) for a long time before they reach supermarket shelves.
But how do producers keep ethylene levels down?
Delay Ripening: Keep fruit cold
The first step for producers to keep fruit from ripening before it reaches you is to pick it at the right time. Once the concentration of ethylene reaches a tipping point of 0.1-1.0 ppm (parts per million) around the fruit, it’s too late to stop it ripening.4
Apples, for example, can be stored for several months after harvest, as long as they’re picked before they start to give off too much ethylene. But once they reach peak ripeness on the tree, they’ll only last about a month after they’re picked, depending on the storage conditions.5
One way to delay ripening is to use cold storage. Low temperatures slow down the reactions inside the fruit that make it ripen. In fact, low temperatures have been used to keep apples crisp since the 1800s, when apples were exported from the U.S. to Europe in wooden barrels in the chilly holds of old shipping vessels.6
High-Tech Ripening: Controlling the atmosphere
Another method to delay ripening is more high tech. Controlled-atmosphere storage (CAS) systems not only regulate the temperature of the room, but can precisely calibrate the levels of different gasses where fruits (like apples) are stored.
Fruit needs oxygen to make ethylene and ripen, so by keeping oxygen levels at 2% – instead of the 21% found in normal air – and increasing carbon dioxide levels, CAS can slow the ripening process right down and essentially put fruits into a kind of hibernation.6
A third way is to use materials that absorb ethylene from the air, like chemical compound potassium permanganate or a synthetic compound called 1-methyl-cyclo-propene (also used to keep cut flowers fresh), to limit the fruit’s exposure to ethylene.1
Once a producer is ready to ship the apples to supermarkets and greengrocers, both ethylene and acetylene – similar chemical compounds – can be used to ripen them, in ripening rooms with controlled temperature and humidity, to make sure they’re crisp and juicy when they reach consumers.7
How to ripen fruit at home
While a lot of technology keeps the fruits in good condition before they reach you, you can take advantage of the same chemistry to help them cross the threshold from almost-ripe to perfect in your fruit bowl at home.
Bananas produce a lot of ethylene, so you can give other fruit, like not-quite-ripe peaches, a helping hand by storing them together. The ethylene from the banana should help ripen the peaches quicker than if it were left to its own devices.
On the odd occasion you may buy some not so ripe fruit, do you have any tips for quickly transforming them into perfectly ripe fruit? Let us know in the comments!